Upgrade to Nix 2.18

[Hoverbear]

Description

Let's update to the new Nix version!

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

🎉

(Do we need / want to update the fixtures?)

[Hoverbear]

Fixtures are there for ensuring we don't accidently break plan format on a patch version, that's it. :)